Extended linear power amplifier systems have been proposed for use in cellular base stations. These systems include a number of linear power amplifier modules producing a corresponding number of output signals that need to be combined into a single high power signal before transmission. Such systems are typically used in communication systems such as in a base site of a radiotelephone system. In such systems, it is desirable that the number of linear power amplifier modules may be any number from a minimum number up to a maximum number of allowable amplifiers. In this manner, the amount of power output by the transmission unit of the base station may be adjusted. In addition, it is desirable that the system provides adequate power amplification efficiency across the entire range of selected amplifiers, i.e. from the minimum number to the maximum number of amplifiers.
However, conventional power combiners, such as Wilkinson type combiners, are only efficient when all of the available amplifiers are present, and rapidly become less efficient due to impedance mismatch and resistive circuit elements when less than the maximum number of amplifiers are selected. For example, a Wilkinson type 10:1 combiner will only be 10% efficient when a single amplifier is coupled to the combiner. This poor efficiency at less than maximum amplification is undesirable for extended linear amplifier applications.
A proposed device for addressing this problem is described in European Patent EP 0540286 A1 ("the '286 patent"). The '286 patent describes a two step impedance transformation process that may be used to design a combiner that has a maximum efficiency at a selectable integer number of installed amplifiers. The integer is selected between the minimum number and the maximum number of amplifiers. Although the described combiner of the '286 patent appears to operate more efficiently than the Wilkinson type combiner, the '286 patent has deficiencies. Forcing the maximum efficiency to be at a selected integer produces sub-optimal performance over a typical range from a minimum to a maximum number of amplifiers since the optimum efficiency location is usually not at an integer value.
Accordingly, there is a need for an improved power combiner that maintains a high efficiency across the entire range of amplifiers that may be coupled to the combiner without degrading the useful bandwidth of the combiner.